A pacemaker is a medical device for implant within a patient that recognizes various arrhythmias such as tachycardia and delivers pacing therapy to the heart in an effort to remedy the arrhythmia. An ICD is a device, also for implant within a patient, which additionally recognizes atrial fibrillation (AF), VT or ventricular fibrillation (VF) and delivers various high voltage electrical shocks to terminate the tachycardia or fibrillation. Within pacemakers and ICDs, it is important to distinguish a tachycardia that arises in the ventricles from those that arise elsewhere in the heart. A tachycardia that arises in the ventricles (referred to as VT) is often more serious than a tachycardia arising elsewhere in the heart, since VT can sometimes lead to VF, which is fatal if untreated. Moreover, the type of therapy to be delivered to the heart depends upon the source of the tachycardia. In particular, it is important to discriminate SVT from VT. SVT is a tachyarrhythmia whose origin is above the ventricles but which is conducted into the ventricles, resulting in an unacceptably rapid ventricular rate. The true underlying arrhythmia in these cases may be, for example, AF, sinus tachycardia (ST), ectopic atrial tachycardia, atrial reentry tachycardia, atrioventricular (A/V) nodal reentry tachycardia, paroxysmal AF or atrial flutter.
Failure to distinguish SVT from VT can result in delivery of inappropriate therapy. Depending upon the capabilities of the implanted device, inappropriate therapy might include the delivery of unnecessary anti-tachycardia pacing (ATP) or the delivery of unneeded high voltage cardioversion shocks. Misidentification of SVT and VT is one of the leading causes of improper device therapy, resulting in the delivery of painful and unnecessary cardioversion shocks.
In particular, problems can arise in devices that automatically deliver high voltage shocks whenever the ventricular rate exceeds a VT rate zone threshold while the observed atrial rate is less than the ventricular rate. In this case, no additional discrimination is ordinarily applied and the arrhythmia is immediately diagnosed as VT with ensuing high voltage therapy. The rationale behind this behavior is that, physiologically, all ventricular tachyarrhythmias where the true ventricular rate exceeds the true atrial rate are considered to be VT. Although this is generally true from a physiological standpoint, there are circumstances of abnormal device behavior that can lead to false VT detections and hence to inappropriate high voltage therapy. For example, the true atrial rate of the patient might be the same as the ventricular rate, but the atrial rate is not properly tracked by the device and is instead underestimated, leading to the false conclusion that the ventricular rate exceeds the atrial rate, with high voltage VT therapy then unnecessarily delivered. These inappropriate shock therapies can significantly affect the quality of life of ICD patients, as well as their morbidity and mortality. Accordingly, reducing inappropriate high voltage shocks that arise in these circumstances is an important problem to solve for patient well being and the invention is generally directed to this end.